A-listbox-in-a-cell-of-a-stringgrid


            Here's another great trick by programmer illusionists. What appears as embedded comboboxes within a stringgrid is actually just a combobox floating above the stringgrid, and it just so happens to be the exact same size as the stringgrid cell underneath it. 
Here's the basics:
 
 procedure TfrmMain.FormCreate(Sender: TObject);
 begin
 StringGrid1.DefaultRowHeight := ComboBox1.Height;
 end;
 procedure TfrmMain.StringGrid1DrawCell(Sender: TObject; Col, Row: 
 Integer; Rect: TRect; State: TGridDrawState);
 var
 R: TRect;
 begin
 if (Col >= StringGrid1.FixedCols) and
 (Row >= StringGrid1.FixedRows) and
 (gdFocused in State) then
 with ComboBox1 do
 begin
 BringToFront;
 CopyRect(R, Rect);
 R.TopLeft := frmMain.ScreenToClient(
 StringGrid1.ClientToScreen(R.TopLeft));
 R.BottomRight := frmMain.ScreenToClient(
 StringGrid1.ClientToScreen(R.BottomRight));
 SetBounds(R.Left, R.Top, R.Right-R.Left, R.Bottom-R.Top);
 end;
 end;
 procedure TfrmMain.StringGrid1TopLeftChanged(Sender: TObject);
 var
 R: TRect;
 begin
 with StringGrid1 do
 CopyRect(R, CellRect(Col, Row));
 with ComboBox1 do
 begin
 Visible := False;
 R.TopLeft := frmMain.ScreenToClient(
 StringGrid1.ClientToScreen(R.TopLeft));
 R.BottomRight := frmMain.ScreenToClient(
 StringGrid1.ClientToScreen(R.BottomRight));
 SetBounds(R.Left, R.Top, R.Right - R.Left, R.Bottom - R.Top);
 end;
 with StringGrid1 do
 if (TopRow <= Row) and (TopRow + VisibleRowCount > Row) then
 ComboBox1.Show;
 end;
 procedure TfrmMain.ComboBox1Change(Sender: TObject);
 begin
 with StringGrid1 do
 Cells[Col, Row] := ComboBox1.Text;
 end;
 
 
In essence, the main routine here is the stringgrid's OnDrawCell event handler. Of course, I also set the stringgrid's DefaultRowHeight property to be the same height as the combobox. In addition, the stringgrid's OnTopLeftChanged event handler is used to hide the combobox when the user scrolls out of view. Also, when the user selects an item from the combobox, simply place the text in the current Col/Row. 
You can also do a couple other little tricks such as setting the stringgrid's Objects[] property to point to the combobox, as well as possibly setting the combobox's Parent property to point to the stringgrid. However, I've had problems with the Parent approach -- namely, that of dropping down the listbox associated with the combobox.
Examples Delphi
